The present invention relates generally to magnetoresistive (MR) read sensors and, more particularly, to an MR read sensor in which an improved antiferromagnetic film provides an exchange-coupled longitudinal bias field in the ferromagnetic MR film of the sensor.
A general description of the principle of operation of MR sensors in magnetic recording systems is provided by Tsang in "Magnetics of Small Magnetoresistive Sensors", Journal of Applied Physics, Vol. 55(6), Mar. 15, 1984, pp. 2226-2231. Essentially, an MR sensor detects magnetic field signals through the resistance changes of the magnetoresistive read element as a function of the amount and direction of magnetic flux being sensed by the element. MR sensors are of interest for three primary reasons: the voltage output when detecting recorded flux transitions in a magnetic medium is large and proportional to an applied sense current; good linear density resolution can be obtained; and the MR sensor output is independent of the relative velocity between sensor and medium.
It is well known in the prior art that in order for an MR sensor to operate optimally, two bias fields are required. To bias the MR material so that its response to a magnetic flux field is linear, a transverse bias field is generally provided. This bias field is normal to the plane of the magnetic media and parallel to the surface of the planar MR element. Typically, the transverse bias field is provided by a current flow through a layer of soft magnetic material deposited adjacent to the MR element and separated by a thin electrically insulting layer.
The second bias field which is typically utilized with MR elements is referred to as the longitudinal bias field and extends parallel to the surface of the magnetic media and parallel to the lengthwise direction of the MR element. The primary purpose of the longitudinal bias is to suppress Barkhausen noise which is generated by multi-domain activities within the MR element. A secondary purpose of the longitudinal bias field is to improve the magnetic stability in the presence of high magnetic field excitation. The longitudinal bias field typically is provided by either hard-magnet or exchange-coupling biasing.
The phenomenon of exchange anisotropy is well-known in the art. It occurs as a result of the interaction of a ferromagnetic material in contact with an antiferromagnetic material, and can be described in terms of an exchange interaction between magnetic moments on each side of the interface between the two materials. For example, exchange coupling between thin layers of nickel-iron (Ni.sub.81 Fe.sub.19) and iron-manganese (Fe.sub.50 Mn.sub.50) produces a unidirectional anisotropy resulting in a shift of the BH loop in the MR element.
Commonly assigned U.S. Pat. No. 4,103,315 to Hempstead et al discloses an MR sensor which utilizes antiferromagnetic-ferromagnetic exchange coupling to provide a uniform longitudinal bias field in the MR element of the sensor. The exchange coupling between the antiferromagnetic and ferromagnetic layers creates a single domain state in the ferromagnetic layer (the MR element) and thereby suppresses the Barkhausen noise associated with domain activity. Hempstead et al teaches an MR sensor in which Ni-Fe serves as the ferromagnetic MR layer and a gamma phase manganese (Mn) alloy with iron (Fe-Mn) having a face-centered-cubic (fcc) structure as the antiferromagnetic layer. Hempstead et al also suggests that alloys of Mn with cobalt (Co), copper (Cu), germanium (Ge), nickel (Ni) and rhodium (Rh) may produce a stable gamma phase Mn alloy when deposited on Ni-Fe.
Commonly assigned U.S. Pat. No. 4,663,685 describes an MR sensor having an Fe-Mn alloy antiferromagnetic layer divided into separate end portions for providing an exchange bias primarily in corresponding end portions of the MR layer.
Two problems encountered with the use of Fe-Mn as the antiferromagnetic layer in MR sensors are its susceptibility to corrosion and the temperature sensitivity of the unidirectional anisotropy field (H.sub.UA). The material is exposed to corrosive environments both during the thin film fabrication processes and during operation of the MR sensor in a magnetic recording system.
In order to decrease the temperature sensitivity of the H.sub.UA, the deposited Ni-Fe/Fe-Mn layers have been annealed for 20-50 hours at a temperature ranging from 260 to 350 degrees C. With such an extended annealing process, significant interdiffusion occurs at the interface between the layers causing the formation of a ternary Ni-Fe-Mn film which provides a much greater H.sub.UA (up to 48 Oe) at the MR sensor operating temperature of 80 degrees C. and exhibits a blocking temperature, at which the H.sub.UA goes to zero, beyond the Neel temperature for bulk Fe-Mn (about 220 degrees C.). The interdiffusion, however, substantially decreases the magnetic moment of the MR sensor. On the other hand, other elements, such as Cr, Ir, Pt, Rh and Ru, for example, have been added to the Fe-Mn alloy layer in order to improve its corrosion resistance. Commonly assigned U.S. Pat. No. 4,755,897 to Howard discloses an improved MR sensor having an antiferromagnetic layer comprising an alloy of Fe, Mn and Cr. In particular, by adding 4.5 at % Cr to a 30 nanometer (nm) thick Fe-Mn film, the corrosion current density (i.sub.c) in an aerated 0.1N sodium sulfate electrolyte decreases from 8 to 2 uA/cm.sup.2, and further decreases to 0.5 uA/cm.sup.2 after annealing the uncoated film in air for one hour at 150 degrees C. However, the addition of the Cr decreases the strength of the H.sub.UA to only 16 Oe at room temperature, and further to about zero at 80 degrees C.